The present application relates to a variable capacitor suitable for application in controlling voltage or current in an electric device or an electronic device, and particularly to an electrode structure of the variable capacitor.
Recently, convenience and efficiency of electronic technology have been appreciated, and the spread of electronic devices typified by IT (information technology) and AV (audio visual) devices has been accelerated worldwide. At the same time, limitation of a global environment and earth resources has been keenly pointed out, and energy-saving technologies for devices have been strongly desired.
For example, power supplies for electronic devices continue to be improved in efficiency, and some switching power supplies have achieved a high efficiency of 90% or more. In an actual situation, however, many low-efficiency power supplies are still used with cost and noise taken into account.
In addition, even high-efficiency power supplies are affected by variations in input power supply voltage, parts variations, and changes in load current, and are greatly decreased in efficiency at a time of low power consumption, for example.
Generally, the power supply is designed to have high efficiency at the rated load (power) of the device. In an actual device, however, operating power is always varying, and the efficiency is also varying at the same time. Taking a television receiver as an example, power to the television receiver is varied greatly depending on audio output and the luminance of the screen. In other words, there is an optimum input voltage for a magnitude of load current.
In addition, the power supply is affected by variations in voltage of a commercial power supply, and therefore has lower power supply efficiency than specifications in actual operation. This is true regardless of whether a power supply system is a switching regulator system or a series regulator system.
FIG. 6 is a diagram showing the efficiency of a transformer. For example, in general, the transformer has a minimum efficiency at a time of no load because a no-load loss 73 occurs even at the time of no load, and thereafter raises efficiency 71 as load current is increased. However, a load loss 72 occurs with the square of the load current. Therefore, when the current exceeds a certain range, the load loss 72 becomes a main factor in a total loss, and thus the efficiency 71 is decreased instead.
In a power supply using the lowering of voltage by a capacitor as shown in FIG. 7, for example, as a power supply that does not actually use a transformer, one terminal of an alternating-current (AC) 100-V commercial power supply 81 is connected to one input terminal of a rectifier circuit 83 formed by a diode bridge via a capacitor 82. Another terminal of the commercial power supply 81 is connected to another input terminal of the rectifier circuit 83. A Zener diode 85 for voltage regulation and a smoothing capacitor 86 are connected in parallel with each other between one output terminal 84a and another output terminal 84b of the rectifier circuit 83.
Such a power supply shown in FIG. 7 that does not use a transformer directly rectifies commercial power 81, and thereafter obtains a stable direct-current voltage (DC) between the output terminals 84a and 84b via the Zener diode 85 forming a regulator.
At this time, the capacitor 82 lowers voltage in advance to lighten a load on the Zener diode 85 forming a regulator.
Thus, the capacitor 82 is often used in the case of low power. This is because with a voltage drop effected by the capacitor 82, the voltage is out of phase with current and therefore no power loss occurs. The capacitor 82 is thus used in a power supply for standby power, for example. However, this circuit varies rectified output according to load variations and the like. Thus, generally, the circuit is configured so as to be adjusted to a maximum load, and power is dissipated by the regulator at a time of light load, whereby stable voltage is created.
In addition, the voltage drop across the capacitor 82 changes greatly according to variations in frequency and load current. Therefore the capacitor 82 cannot be used in a device involving a high load current and great load variations. The capacitor 82 is thus limited to use for a very low power of a few ten mW such as standby power or the like in the present situation.
The power supplied by the power supply of FIG. 7 that does not use a transformer can be increased by connecting another predetermined capacitor in parallel with the capacitor 82 at a time of an operation consuming high power through a relay or the like. However, to deal with a wide load range requires switching of the plurality of capacitors.
The switching of the plurality of capacitors through the relay or the like is possible in principle, but is not practical in consideration of not only space and cost but also slow response, occurrence of noise at a time of switching, inability to change capacitance continuously, and poor durability. Therefore a device is required which can continuously change the capacitance value according to load variations.
Incidentally, there is for example a varicap using a capacitance between terminals of a diode as a capacitor whose capacitance can be electrically controlled for use in a high-frequency circuit. However, the varicap cannot be used for power control because the capacitance value, withstand voltage and the like of the varicap are not suitable.
Generally, the capacitance of a capacitor, including for example variable capacitors using a MEMS (microelectromechanical system) in recent years, is determined by a dielectric constant, electrode area, and a distance between electrodes. Therefore it suffices to control one or more of them. Control of capacitance of a capacitor which control is actually proposed in MEMS is a method of changing a distance between electrodes or an opposed electrode area by displacing an electrode.
Patent Document 1 (Japanese Patent Laid-open No. Sho 62-259417) discloses an example of changing capacitance by 70% by applying 50 V to a ceramic capacitor and thereby changing a dielectric constant. Changing the cutoff frequency of a filter circuit or the oscillation frequency of a time constant oscillator circuit is proposed as an application.
A power loss in an electronic device or an electronic circuit as described above leads not only to an increase in power used and extra electricity charges borne by users but also to a waste of earth resources and acceleration of global warming. It is desirable that the power loss be minimized.
In a series regulator system using a power transformer with a simple circuit and low noise, voltage is lowered to a necessary voltage by the power transformer connected to a commercial power supply, thereafter rectified by a diode, and then smoothed by a large-capacitance capacitor. The rectified output is unstable, and therefore the voltage is stabilized by a regulator for controlling a voltage drop between terminals of a transistor.
The voltage drop in this case is a direct-current voltage drop, and is basically all converted into heat, so that a great power loss occurs. A necessary amount of the voltage drop is greatly affected by variations in characteristics of the power transformer and other parts, the magnitude of load current, and the like. When a margin is provided for stable operation of the electronic device, a power loss is greatly increased in a normal state, and results in an efficiency of 30% in extreme cases.
In a switching regulator system, voltage is stabilized by on-off control of a semiconductor element. Therefore the power loss is small and a high efficiency is obtained. Even so, the efficiency is changed according to input or load conditions, and the efficiency is degraded under a light-load condition or the like. There is accordingly a desire to deal with a wider range of input and load variations.
Thus, for power use, a variable capacitor device having control terminals for varying capacitance, having large capacitance, having high withstand voltage, and permitting a high current is required, and high reliability is further required.
In this variable capacitor, normally, control voltage is constant and no current flows in principle. Although a transient current flows when the control voltage is changed, the value of the current is very low. A great advantage of this power controlling device lies in such minimum power being required for control. Hence, required specifications differ greatly between electrodes for control and electrodes for input and output, and thus consideration needs to be given to reliability and productivity in manufacturing.